The present invention generally relates to a copying apparatus and more particularly, to construction of a developing means employed for the copying apparatus.
Commonly, in a copying apparatus, for example, a reader printer arranged to effect reading and copying of microfilms, depending on whether the microfilm for an original is of a positive image or a negative image, there are required one system for developing and transferring the positive image as it is, and another system for effecting developing and transferring by inverting the negative image into a positive image.
For meeting the requirements as described above, there have conventionally been employed practices as follows.
(i) A practice in which, through employment of a photosensitive member having sensitivites for both polarities, it is made possible to effect the reversal development by altering the polarity of an electrostatic latent image formed on the photosensitive member.
(ii) A practice in which, by employing electrically conductive magnetic toner, the reversal development is made possible through alteration of a bias voltage.
However, in the known practices as described above, there are disadvantages such that the practice (i) requires the employment of the special photosensitive member, while in the practice (ii), the developed toner image can not be transferred onto a plain paper.
Accordingly, in order to make it possible to transfer developed toner images onto a plain paper sheet through employment of a generally used photosensitive member, without alteration of the charging characteristics thereof, another practice (iii) may be considered, in which two developing means are provided so as to separately accommodate therein positively charged toner and negatively charged toner for effecting selective development by either one of the toners.
However, the above practice (iii) also has a disadvantage as stated below.
(a) The practice is uneconomical, since the toner removed from the photosensitive member after the transfer can not be reused through recycling due to the employment of two kinds of toner.
(b) Owing to the fact that two toner hoppers are provided side by side and the two kinds of toner are both black, there is a possibility that the toner to be replenished or the hopper for the replenishment will be wrongly selected by the operator.
(c) When the fixing conditions are taken into account, two kinds of toner having exactly the same characteristics except for the charging polarity are required, and therefore, selection and manufacture of the toner are difficult.